EUV light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays) and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has an element, e.g., xenon, lithium or tin, indium, antimony, tellurium, aluminum, etc. with an emission line in the EUV range. In one such method, often termed laser-produced plasma (LPP) the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam.
Once generated, the EUV light is typically reflected by a multi-layer mirror, sometimes called a collector mirror. For example, in one setup, a normal incidence elliptical reflector may be used having an aperture to allow laser light to pass through and reach the target material at an irradiation site. In one arrangement, a collector in the shape of a prolate ellipsoid may be positioned such that its first focus is located at the irradiation site and its second focus is positioned at a so-called intermediate point (also called the intermediate focus) where the EUV light may be output from the light source and input to, e.g., an integrated circuit lithography tool.
Some lithography tools utilize an arc field illumination field to efficiently irradiate the tool's photomask/reticle. For example, see U.S. Pat. No. 6,210,865 entitled “EXTREME-UV LITHOGRAPHY CONDENSOR” which issued to Sweatt et al on Apr. 3, 2001, the contents of which is hereby incorporated by reference herein. Thus, for this type of tool, the EUV light generated at the plasma irradiation site may need to be collected, condensed and shaped to create the arc field. Typically, for EUV light, reflective optics, e.g., grazing and/or normal incidence mirrors, are used, with each reflection resulting in an in-band intensity loss of about 20-40%. Thus, it may be desirable to use as few optics as possible between the plasma irradiation site and the photomask/reticle.
Another factor that is often considered when designing a high volume EUV light source is the generation and mitigation of debris which may damage EUV light source optics such as a laser input window, collector mirror and/or metrology equipment. Thus, for at least some source materials, the production of a plasma may also generate undesirable by-products in the plasma chamber, (e.g., debris) which can potentially damage or reduce the operational efficiency of the various plasma chamber optical elements. This debris can include out-of-band photons, high energy ions and scattered debris from the plasma formation, e.g., atoms and/or clumps/microdroplets of source material. This debris may also include chamber material from secondary sputtering and for the case of electric discharge type systems, electrode material. For this reason, it is often desirable to employ one or more techniques to minimize the types, relative amounts and total amount of debris formed for a given EUV output power. When the target size, e.g., droplet diameter, and/or target makeup, e.g., chemistry, are chosen to minimize debris, the targets are sometimes referred to as so-called “mass limited” targets.
The high energy ions and/or source material debris may be damaging to the optical elements in a number of ways, including heating them, coating them with materials which reduce light transmission, penetrating into them and, e.g., damaging structural integrity and/or optical properties, e.g., the ability of a mirror to reflect light at such short wavelengths, corroding or eroding them and/or diffusing into them. Thus, debris reduction and/or suitable techniques to reduce the impact of debris may need to be considered in the design of a high volume EUV light source.
One way to reduce the influence of debris is to move the collector mirror away from the irradiation site. This, in turn, implies the use of a larger collector mirror to collect the same amount of light. The performance of a collector mirror, e.g., the ability to accurately direct as much in-band light as possible to, e.g., a focal point, depends of the figure and surface finish, e.g., roughness of the collector. As one might expect, it becomes more and more difficult to produce a suitable figure and surface finish as the size of the collector mirror grows. Typically, these EUV collector mirrors have included a monolithic substrate overlaid with a multilayer dielectric coating, e.g., Mo/Si. Depending on the application, these multilayer mirrors may also include thin barrier layers deposited at one or more interfaces and in some cases can include a capping layer. Collector mirror substrate requirements may include one or more of the following: vacuum compatibility, mechanical strength, e.g. high temperature strength, high thermal conductivity, low thermal expansion, dimensional stability, ability to be polished to a suitable figure and finish, and the ability to be brazed or bonded.
With the above in mind, Applicants disclose EUV optics including collector mirrors, corresponding fabrication methods, and methods of use.